The invention relates to a method of testing containers, more particularly glass or plastic bottles, involving placing a head sealingly onto the mouth of the container, putting the interior of the container under pressure by means of a metering piston which can be displaced in relation to the head, and measuring the pressure by means of a pressure sensor to check the volume of the container.
The invention further relates to an apparatus for implementing the method, comprising a head adapted to be moved sealingly onto a mouth of the container, a metering piston which is displaceable in relation to the head and can be used to pressurize the interior of the container, and a pressure sensor for measuring the pressure in the interior of the container.
One such apparatus is known in conjunction with an inspection machine for plastic bottles from DE 37 22 422 C2. In the case of this known apparatus the bottle can be clamped on a plate by means of a head fastened in freely rotatable manner to a vertically displaceable carriage. The interior of the bottle can be pressurised through the head, said pressure being recorded by a pressure sensor disposed on the head. The interior of the container is pressurised using a metering piston adapted to be displaceable at right angles to the head in a cylinder provided inside the carriage. A piston rod joined to the metering piston is able to be displaced against the pressure of a return spring by a cam disc attached to the machine frame via a follower roller. On the side of the metering piston nearest the return spring is a cylinder chamber. Whenever the metering piston is displaced, the air in this cylinder chamber is forced into the bottle along a bore provided in the head, and the pressure sensor records the pressure that builds up inside the bottle. An electronic analyser compares this pressure with a limiting value and subsequently actuates an ejector in the event of excessive deviation from this limiting value. A bottle of nominal volume has a given limiting pressure value. If the volume of the bottle is smaller or greater than the nominal volume, the limiting pressure value is respectively exceeded or not attained when the said volume of air is forced in.
The aforesaid inspection machine is primarily intended for inspecting returnable bottles made from plastic, ie. bottles returned by the customer and then refilled. The known apparatus works perfectly satisfactorily on this type of inspection machine. However, it has been shown that problems can arise if the apparatus is used on an inspection machine employed for series production of PET or glass bottles. Volume deviations may even occur in this sort of series manufacturing process. It is therefore necessary to check the volume of the bottles using an apparatus of the type indicated initially. If the volume deviations are outside a given tolerance, the bottles concerned again have to be identified as defective and removed. In this type of series production process the bottles, which have usually come from a blow moulding machine, contain warmed air. The volume of air forced into the bottle using the metering piston is relatively small in comparison to the volume of the bottle, and the excess pressure produced by forcing in the volume of air is correspondingly small. If in the series production process the measurement result is also affected by volumes of air at different temperatures, it is no longer possible to obtain reliable readings. Furthermore, it is in any event no simple matter to provide an external volume of air that is always constant and ideally at the same temperature and force it into a bottle, for in series production there is usually less than a second available for the entire check per bottle.
It has been found that fixing the start of the pressure measurements is a difficult matter. Yet a precise starting point for the pressure measurements is indispensable, for as a rule it is necessary in the very brief time available for measurement to take a plurality of pressure measurements under various defined measurement conditions.